This invention relates to travelling wave tubes.
Conventional travelling wave tubes employ cathodes which are indirectly heated by radiation or conduction from a heating element. Such cathodes tend to be of high thermal mass and the time taken for the cathode to attain operating temperature from switching on of the heater element tends to be relatively long. In many cases this is not an inconvenience. For certain applications, however, it is highly desirable to reduce the time taken for the cathode to attain operating temperature and one object of the present invention is to provide an improved travelling wave tube of which the cathode exhibits relatively rapid start characteristics.